1. Technical Field of the Invention
This invention relates to isotopic analysis associated with oil and gas drilling operations and, more particularly, to an interpretive method of a novel mud gas isotope logging technique.
2. Description of Related Art
Within the oil and gas industry, laboratory analysis of gas samples obtained during a drilling operation can be employed to determine geochemical information associated with strikes of oil or gas deposits. The laboratory analysis may include the acquisition of compositional and isotopic data of sampled subsurface gases. The data is applied to traditional geochemical plots and templates as contained in scientific literature. The interpretation of this data is used to provide geochemical information on where the gas provenance may have originated from (“source rock”), how thermally mature the gas is (how hot the source got before expelling gas), whether subsurface post-generation effects (pressure, volume, temperature (PVT) effects, biodegration, water-washing, etc.) were encountered during migration of the gas from the source rock to a reservoir, and any problems or effects the hydrocarbons in the reservoir subsequently experienced.
Existing well sampling techniques use physical gas samples for compositional and isotopic laboratory analysis. There are typically three ways that gas samples may be obtained. First, gas can be sampled directly from the reservoir formation of interest using a logging tool, such as a modular dynamic tester (MDT) or a repeat formation tester (RFT). To use these logging tools, the entire drill bit and string (tubing) has to be removed before the logging tools can be sent back down to the formation interval of interest in order to obtain a physical sample. Since drilling operations must be stopped while using these logging tools, the logging tools are used sparingly and limited sample numbers are collected due to the expense incurred in delaying drilling operations. Another way to obtain gas samples is by “canned cuttings.” Rock samples (“cuttings”), representative of a subsurface formation, are pulverized by a drill bit as the bit penetrates rock strata. The rock samples are then collected in sealed cans upon return to the surface in the circulating mud stream where the cuttings are able to “degas.” Gases accumulating in the sealed cans can then be analyzed in a laboratory as “headspace gases.” The cuttings are collected/suspended in the mud stream that is continuously circulated around the drill bit during drilling. The mud stream is employed to help lubricate and cool the drill bit and prevent rock material from accumulating and collecting around the drill bit. Because the cuttings vary in size and density according to the rock material being drilled and type of drill bit, the heavier and more dense material is likely to sink below smaller, less dense material as the mud is circulated up to the surface. Therefore, these cuttings “smear” in the mudstream as they circulate toward the surface. To compensate for this, canned cuttings are usually collected over a large depth interval (typically ninety feet) in an attempt to collect as representative a sample as possible. The third way that a sample is collected is by directly sampling gases entrained in the mud system during drilling. As a well drill bit penetrates and pulverizes rock material in its path, free and absorbed gases entrained in the pulverized rock and immediately adjacent rock formation (side of the borehole) flow into the mud stream as it circulates around the drill bit. These gases are carried to the surface and collected as they exsolve/degas from the returning mud stream.
Standard mud gas chromatographic compositional analyses and interpretations suffer from several disadvantages. None of the analyses effectively detail or correlate geological information such as seals and barriers (hydrocarbon communication and compartmentalization problems), good communication zones, or gas diffusion into their interpretation. Data can result in false positives and negatives where changes in operational drilling conditions related to variables such as increased rate of penetration (ROP) or mud weight increases occur. A more advanced method is needed which employs an integrated interpretation and approach of drilling, geological and engineering information together with mud-gas chromatographic compositional and isotopic analysis.
Typically, existing methods merely employ geochemical data to provide geochemical interpretations. The present invention employs geochemical data to provide geochemical, geological and engineering interpretations and solutions.
Thus, it would be a distinct advantage to have an interpretive method of analysis of mud-gas samples utilizing chromatographic compositional and isotopic analysis concomitant with a geochemical, geological, and engineering interpretation applied to the data. It is an object of the present invention to provide such an interpretative method specific to mud gas isotope logging.